Our goal is the development of methods for 1) the specific isolation of antigen-reactive cells for the study of the molecular interactions that occur in their immune reactions, and 2) their application to problems of tumor immunology. In order to be able to directly measure binding of antigen-specific receptors to MHC molecules, we have accomplished the synthesis of conjugates of anti-MHC antibodies and water-soluble polymers. In order to study targeting of tumors with MHC antigens, we have synthesized, by crosslinking different monoclonal antibodies, heterconjugates that can bind two different class I MHC antigens. In studies aimed at understanding the enhanced immunogenicity of a xenogenized tumor cell line, we have found that the immune response to it is MHC restricted at the level of both helper and cytotoxic T cells. Furthermore, the xenogenized cell line, unlike the unmodified, unrejected tumor line from which it was derived, was found to have features characteristics of antigen presenting cells, i.e. surface expression of class II (Ia) antigens, and secretion of material with IL1-like activity.